1. Field of the Invention
The present invention relates to a micromachine production method, and more particularly, to a micromachine having a frequency filter function and being capable of integrating into a semiconductor device, and a production method thereof.
2. Description of the Related Art
A microvibrator fabricated using a semiconductor process technology has features to ensure that an area occupied by each element is of small size, high quality factors (Q factors) are available and integration with other semiconductor devices is possible. Research organizations including University of Michigan have proposed to use the microvibrator as radio communication devices such as IF filters and RF filters, for instance. See Non-patent document 1, for instance.
A conventional technology of a high frequency filter for radio communication is now described with reference to FIG. 1. As shown in FIG. 1, a microvibrator 101 has a configuration as described below. A vibrator electrode 113 is arranged, through a space 121, above an output electrode 112 provided on a substrate 111. An input electrode 114 is connected to the vibrator electrode 113.
Next, operations of the microvibrator 101 are described below. When a specific frequency voltage is applied to the input electrode 114, a beam (a vibration part) 113A of the vibrator electrode 113 arranged above the output electrode 112 through the space 121 vibrates at a natural vibration frequency. A capacitance of a capacitor formed by the space 121 between the output electrode 112 and the beam (the vibration part) 113A is changed by the vibrations of the beam 113A, and this is outputted from the output electrode 112 as a voltage (See the Non-patent document 1, for instance). A high frequency filter comprising the microvibrator 101 having the above configuration enables higher Q factors to be obtained, as compared with a high frequency filter utilizing Surface Acoustic Wave (which will be hereinafter referred to as SAW) or Film Bulk Acoustic Resonator (which will be hereinafter referred to as FBAR).
A method of producing the above-described microvibrator is exemplified with reference to FIGS. 2A to 2E.
As shown in FIG. 2A, a silicon oxide film 212, a silicon nitride film 213 and a silicon oxide film 214 are sequentially formed at a silicon substrate 211. Next, a trench 215 that is to form an output electrode is formed at the silicon oxide film 214 using ordinary lithography and etching technologies.
Next, as shown in FIG. 2B, a polysilicon film 216 that works as the output electrode is formed at the silicon oxide film 214 so as to fill up the trench 215. Thereafter, the polysilicon film 216 on the silicon oxide film 214 is polished using Chemical Mechanical Planarization (CMP) to be an output electrode 217 that is obtained with the polysilicon film 216 left only in the trench 215, as shown in FIG. 2C.
Next, as shown in FIG. 2D, a silicon nitride film 218 that covers the output electrode 217 is formed on the silicon oxide film 214. Then, an opening 219 is formed in the silicon nitride film 218 on the output electrode 217 using the lithography and etching technologies. In addition, a sacrifice film 220 comprising a silicon oxide film is formed so as to fill up the opening 219. At this time, planarization of the sacrifice film 220 is carried out using polishing or the like so that the sacrifice film 220 may have the same surface as that of the silicon nitride film 218. Thereafter, a vibrator electrode 221 is formed so as to cross the sacrifice film 220.
Next, as shown in FIG. 2E, the sacrifice film 220 (refer to FIG. 2D described above) is removed selectively by etching to form a space 222 between the vibrator electrode 221 and the output electrode 217. Thereafter, an input electrode (not shown) that is to be connected to the vibrator electrode 221 is formed. In this manner, a microvibrator 201 is fabricated.
[Non-patent document 1]
C. T. -C. Nguyen, “Micromechanical components for miniaturized low-power communications (invited plenary),” proceedings, 1999 IEEE MTT-S International Microwave Symposium RF MEMS Workshop, Jun. 18, 1999, P.48-77.
However, in the above-described conventional production method, when forming the output electrode, removal of the polysilicon film on the silicon oxide film is carried out using polishing, after the polysilicon film has been buried in the trench. In this process, ideally, surface planarization is attained to permit a flat output electrode to be formed. However, practically, large dishing arises so that an output electrode of a desired shape is not obtainable. More specifically, as shown in FIG. 3, the dishing that causes the surface of the polysilicon film 216 to be largely shaved arises. The above dishing also causes the silicon oxide film 214 around the trench 215 to be shaved. When the above dishing arises, a space to be formed between the output electrode and the vibrator electrode is hardly formed according to a design value, and as a result, desired frequency characteristics are not obtainable. Particularly, when forming an output electrode having a width of several μm or more, the polysilicon film 216 is not allowed to remain as it is in a condition where the trench 215 is completely filled up with the polysilicon film 216. The above-described dishing problem is further growing particularly with an increase in a wide output electrode.
Alternatively, to suppress the dishing of the polysilicon film, there may be used a different production method, wherein before or after formation of a trench that is to form an output electrode, a polishing stopper film comprising a silicon nitride film is formed on a silicon oxide film, and a polysilicon film that works as the output electrode is then buried in the trench. However, as shown in FIG. 4, even if a polishing stopper film 231 comprising the silicon nitride film is formed on the silicon oxide film 214, the dishing also arises at the time when the polysilicon film 216 is polished.
An extent of dishing of the polysilicon film in the presence of the polishing stopper film 231 is more reduced than that of the polysilicon film in the absence of the polishing stopper film 231 shown in FIG. 3. However, the above polishing stopper film is not sufficient to suppress the dishing, the space to be formed between the output electrode and the vibrator electrode is thus hardly formed according to the design value, and as a result, desired frequency characteristics are not obtainable.
Even if over-polishing has not been applied to remove the polysilicon film on the silicon oxide film completely, it was difficult to suppress the dishing in terms of characteristics of a pad or slurry used in polysilicon polishing.